I planted some lovely pansies the other day, right under my
front window. It was 80 degrees out the day I planted, and
the
little guys have been doing quite well since transplanted.

But oh no, tonight's weather man predicts a possible frost
overnight, and I am suddenly terrified about the
poor
pansies
prospects of the possibility of getting covered in frost.

Hence the immediate need for hoodies for each of my
fifteen
Pansies that I planted prematurely, perhaps.

Made from a fine, silk like fabric that keeps the frost from
landing directly on the poor delicate flowers. These teeny
hoodies will surely save my babies from the torture of
frostbite. Purplish, is the color I had in mind, it would go
well
with their coloring. Yes.

I think you will find that frost doesn't "land on"
anything, any more than dew lands on something, or
condensation on the outside of a glass of ice-water
"lands on" the outside of the glass. All three have
something in common, though....

An item exposed to cloudless sky at night tends to
radiate heat easily, and becomes colder than the air
temperature. That coldness causes condensation of
atmospheric humidity. If it gets even colder, the
condensation will freeze.

So, all you need is a kind of cover over the whole flower
bed, to reflect radiation back down to the flower bed.
Try some "space blanket" material.

//An item exposed to cloudless sky at night tends
to radiate heat easily, and becomes colder than
the air temperature. // I agree. If I have a low
thermal mass object (say a leaf, not a rock) that
has low thermal conduction to the ground (which
is at say 2°C), in still air that is at 1°C, and it is a
cloudless night, the air, which is fairly transparent
to infrared light, may not be cooled by radiation as
much as the leaf.

I found several statements online supporting this,
but didn't find any that were "authoritative".
Several of the site that I thought might be
considered authoritative (Wikipedia, Cornell
extensions) mentioned cooling through radiation
but stressed the settling of cold air near the
ground to explain frost forming on the ground
when the air temperature a few feet above the
ground is above freezing. However I would like to
point out that water will not condense on an
object that is warmer than the surrounding air. If
an object cools below air temperature due to
evaporation, that indicates that the air must be
dry enough that dew/frost is not going to happen.
Typically the ground is warm compared to the air
at night. Therefore the only possible explanation
for frost or dew is that radiation makes the leaves
colder than the air.

//Therefore the only possible explanation for frost or dew is
that radiation makes the leaves colder than the air.//

To quote a Yorkshire acquaintance, "ah don't bloody believe
it."

For one thing, heat transfer by conduction will overwhelm
heat transfer by radiation, at those sorts of temperatures.

For another, everything around the leaf (or whatever) will
be emitting radiation corresponding to its temperature.

Try the following thought experiment. Take a [dry]
thermometer outdoors on a clear night, and wait until it
gives a steady reading. Now take a piece of black card
(which you have allowed to cool to ambient temperature)
and hold it a couple of feet above the thermometer to
obscure its view of the night sky. Does the thermometer
show an increase in temperature?

// Take a [dry] thermometer outdoors on a clear
night, and wait until it gives a steady reading.
Now take a piece of black card (which you have
allowed to cool to ambient temperature) and hold
it a couple of feet above the thermometer to
obscure its view of the night sky. Does the
thermometer show an increase in temperature? //

Yeah. That's why the car under the carport
doesn't get frost on it while the one outside the
carport needs to have its windows scraped. It
would be interesting to do the experiment with a
thermometer too though.